Studies of microstructure and mechanical properties of 6xxx alloys for automotive applications

Abstract

The aluminium alloys are widely used in the automotive sector since their main feature is a significant improvement of mechanical properties through different strengthening mechanisms. In particular, the 6xxx aluminium alloys a combination of high strengthening potential and low specific weight. This characteristic becomes clearly fundamental nowadays to reduce the weight of vehicles and consequently reduce the CO2 emissions. The effects of different chemical compositions and treatments on microstructure and final mechanical properties have been widely studied, but to date there are still questions to be addressed to have a clear idea of the topic and engineer microstructures with favourable mechanical properties. In this study a deeper understanding of these effects is attempted. Two high strength 6xxx aluminium alloys, made by extrusion and provided by Constellium, were investigated. Different alloys were prepared changing the chemical composition and the thermomechanical process. The work has been divided in three main steps related to the study of alloys after extrusion, alloys subjected to conventional heat treatment (T6) and alloys subjected to thermomechanical treatment (aDA). In the first step, the role of Zr addition and homogenisation temperature on the recrystallisation phenomenon was investigated by studying their effects on intermetallic phases and yield strength of the material. Different alloys were considered changing homogenisation temperatures and Zr addition. Moreover, the microstructure was analysed inside and outside the peripheral coarse grain (PCG) zone in the extruded profile to observe any differences in intermetallics which can be linked to the recrystallisation phenomenon. Optical microscopy, SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) were used to analyse the microstructure. EDX and SADP were used to identify the intermetallic phases. In the second step, the study was focused on the role of Cu addition and time of natural ageing in changing the precipitation sequence and so the effects of precipitation hardening. Four variants of Al-Mg-Si alloys were studied changing the Cu addition (high and low content) and the time of natural ageing (immediate and delayed natural ageing changing the period between extrusion and artificial ageing). The microstructure has been studied using TEM focusing on the role of precipitates in improving the mechanical properties. The identification of precipitates has been conducted by SADP and HRTEM. In the last step, the study was focused on the combination of precipitation hardening and work hardening and how they are affected by Cu addition and time of natural ageing. The treatment used involves three phases: a pre-ageing, a deformation, and a final-ageing. The same variants of alloys were studied as the previous section changing the Cu addition and the time of natural ageing. The microstructure has been studied using TEM focusing on the role of precipitates and dislocations in improving the mechanical properties and using atom probe to study clusters in the microstructure. STEM was used to study the dislocations configuration. In all the steps, the features of microstructures have been correlated to the mechanical properties of the material in order to fully understand their effects. The mechanical properties were obtained through standard tensile testing and hardness measurements. The experimental results showed that the yield strength increased with lower homogenisation temperature and higher amount of Zr, while the depth of PCG followed the opposite trend. The decrease of homogenisation temperature led to a microstructure with fine and highly dispersed intermetallics, while the increase in Zr addition did not influence the size of intermetallics but was related to an increase in their density. The EDX analysis and the analysis of diffraction patterns revealed the presence of Mn-based and Zr-based intermetallics. These results showed a correlation between the intermetallics present in the microstructure and the mechanical properties of material. Regarding the Cu addition and the time of natural ageing, it was revealed that the increase of Cu addition and the reduction of time of natural ageing led to beneficial effects on yield strength and hardness for both T6 and aDA alloys. This is correlated to the microstructures obtained. All the alloys showed the presence of β’’ and Q’ precipitates. The T6 microstructures with best mechanical properties showed a higher number density of precipitates and smaller width of PFZ (Precipitate-Free Zone) exhibiting a better response to precipitation hardening. The yield strength and the hardness of aDA material increased throughout the process with the highest values obtained after the final ageing. The microstructure after pre-ageing and after deformation did not show the presence of precipitates at the same resolution used for the microstructure after final ageing (50-100 nm). The analysis on dislocations have shown an increase of dislocations density decreasing the Cu addition showing a better response of the alloys with low Cu addition to work hardening. The material subjected to aDA treatment showed an improvement in mechanical properties compared to the one realised with T6 treatment and this confirmed the beneficial effects of thermomechanical treatment.